1. Field of the Invention
This invention relates generally to optical devices and, in particular, to vertical-cavity surface-emitting lasers (VCSELs).
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Lasers have a wide range of industrial and scientific uses. There are several types of lasers, including gas lasers, solid-state lasers, liquid (dye) lasers, and free electron lasers. Semiconductor lasers are also in use. The possibility of amplification of electromagnetic waves in a semiconductor superlattice structure, i.e., the possibility of semiconductor diode lasers, was predicted in a seminal paper by R. F. Kazarinov, et al., xe2x80x9cPossibility of the Amplification of Electromagnetic Waves in a Semiconductor with a Superlattice,xe2x80x9d Soviet Physics Semiconductors, vol. 5, No. 4, pp. 707-709 (October 1971). Semiconductor laser technology has continued to develop since this discovery.
There are a variety of types of semiconductor lasers. Semiconductor lasers may be diode lasers (bipolar) or non-diode lasers such as quantum cascade (QC) lasers (unipolar). Semiconductor lasers of various types may be electrically pumped (EP) (by a direct or alternating current), or pumped in other ways, such as by optically pumping (OP) or electron beam pumping. Semiconductor lasers are used for a variety of applications and can be built with different structures and semiconductor materials, such as gallium arsenide (GaAs).
Semiconductor lasers are typically powered by applying an electrical potential difference across the active region, which causes a current to flow therein. Electrons in the active region attain high energy states as a result of the potential applied. When the electrons spontaneously drop in energy state, photons are produced (to carry away the energy lost by the transition, so as to conserve energy). Some of those photons travel in a direction perpendicular to the reflectors of the laser. As a result of the ensuing reflections, the photons can travel through the active region multiple times.
Stimulated emission occurs when an electron is in a higher energy level and a photon with an energy equal to the difference between the electron""s energy and a lower energy interacts with the electron. In this case, the photon stimulates the electron to fall into the lower energy state, thereby emitting a photon. The emitted photon will have the same energy as the original photon, and, if viewed as waves, there will be two waves emitted (from the electron""s atom) in phase with the same frequency. Thus, when the photons produced by spontaneous electron transition photons interact with other high energy state electrons, stimulated emission can occur so that two photons with identical characteristics are present. If most electrons encountered by the photons are in the high energy state, the number of photons traveling between the reflectors tends to increase, giving rise to amplification of light and thus lasing.
The use of semiconductor diode lasers for forming a source of optical energy is attractive for a number of reasons. For example, diode lasers have a relatively small volume and consume a small amount of power as compared to conventional laser devices. Further, the diode laser is a monolithic device, and does not require a combination of a resonant cavity with external mirrors and other structures to generate a coherent output laser beam.
Additionally, semiconductor lasers may be edge-emitting lasers or surface-emitting lasers (SELs). Edge-emitting semiconductor lasers output their radiation parallel to the wafer surface, while in SELs, the radiation output is perpendicular to the wafer surface.
One type of SEL is the vertical-cavity surface-emitting laser (VCSEL). The VCSEL structure usually consists of an active (optical gain) region sandwiched between two distributed Bragg reflector (DBR) mirrors: a top, exit DBR, and a bottom DBR. DBRs are sometimes referred to as mirror stacks. The DBR mirrors of a typical VCSEL can be constructed from dielectric or semiconductor layers (or a combination of both, including metal mirror sections). DBRs or mirror stacks in VCSELs are typically formed of multiple pairs of layers often referred to as mirror pairs. The pairs of layers are formed of a material system generally consisting of two materials having different indices of refraction and being easily lattice matched to the other portions of the VCSEL. The number of mirror pairs per stack may range from 4-60 pairs to achieve a high percentage of reflectivity, depending on the difference between the refractive indices of the layers. A larger number of mirror pairs increases the percentage of reflected light (reflectivity).
When properly designed, these mirror pairs will cause a desired reflectivity at the laser wavelength. Typically in a VCSEL, the mirrors are designed so that the bottom DBR mirror (i.e. the one interposed between the substrate material and the active region) has nearly 100% reflectivity, while the top DBR mirror has a reflectivity that may be 98%-99.5% (depending on the details of the laser design). Of course, various laser structures may vary from these general properties.
High reflectivity (approaching 100%) at the bottom DBR mirror is generally desired in a VCSEL for two reasons. First, any portion of the optical field that xe2x80x9cleaksxe2x80x9d out the back of the bottom DBR mirror represents a power loss that reduces efficiency. This reduced efficiency may be so great so as to prevent the laser from operating at all (i.e. the efficiency goes to zero). A second reason why a nearly unity reflection coefficient is desired for the bottom DBR mirror is related to the issue of optical feedback into the laser cavity.
VCSELs have many attractive features such as low threshold current, single longitudinal mode, a circular output beam, among others. However, problems of polarization stabilization remain unsolved because of the isotropy of the gain of VCSELs. Because of the circularly symmetric design of the VCSEL cavity, the VCSEL polarization direction is not limited. Therefore, the polarization direction of the emitted light is random and is easily switched due to stress, injected current, or reflected light. Directly modulating a VCSEL can lead to severe polarization noise, for example, partly because under modulation, the polarization extinction ratio decreases. Thus, the absence of a well-defined polarization selection mechanism leads to the coexistence, switching, or bistability of lasing modes and even to the rotation of the polarization eigenstates (0 degree and 90 degree polarization) with a change in the drive current or temperature. When the dominant polarization is different for different transverse modes, mode partition will cause fluctuations in the polarization state of the VCSEL output.
Polarization instability is undesirable in many applications. Polarization control would be useful in applications such as optical free-space routing and in a polarization-duplicated transmitter, or optical head. For example, free-space systems frequently rely on polarization-sensitive, diffractive optical elements, such as holograms, for splitting and routing of various optical information channels, or other types of beamsplitters. Additionally, polarization needs to be well controlled to avoid polarization-induced noise caused by the unstable polarization states. Polarization-stabilized VCSELs can show significantly reduced transient partition between orthogonally polarized lasing modes, for example. Efforts have been made to control polarization by using differences in sidewall reflectivity or stress from an elliptical window hole. Also, an asymmetrically designed active layer has been proposed. However, a method to effectively control polarization has not yet been developed. For example, a metal grating could be added to polarize a laser""s output, but this could cause scattering. These and related matters are discussed in Vertical-Cavity Surface-Emitting Lasers: Design, Fabrication, Characterization, and Applications, edited by Carl W. Wilmsen, Henryk Temkin, and Larry A. Coldren (Cambridge University Press, 1999), particularly sections 6.1, 6.3, 10.1, and 10.2, which are incorporated herein by reference; also J. H. Ser et al., xe2x80x9cPolarization Stabilization of Vertical-Cavity Top-Surface-Emitting Lasers by Inscription of Fine Metal-Interlaced Gratings,xe2x80x9d Appl. Phys. Lett., Vol. 66, No. 21, May 22, 1995, pp. 2769-2771, which article is incorporated herein by reference.
There is therefore a need for improved VCSELs having a well-defined polarization selection mechanism.
The present invention is directed to a VCSEL and method of fabricating same. First, a substrate is provided. Then, a first reflector is disposed (where xe2x80x9cdisposedxe2x80x9d includes being deposited or epitaxially grown) on the substrate, which is followed by an active region being disposed on the first reflector. Then, a second reflector is disposed on the active region such that the active region is interposed between the first reflector and the second reflector. Then, a polarizer is formed inside the second reflector. The polarizer contains parallel stripes of material doped differently than that of the other parts of the second reflector. The polarizer polarizes the light generated from the active region.
Another embodiment of the present invention is directed to a VCSEL that includes a substrate, a first reflector disposed on the substrate, an active region disposed on the first reflector, and a second reflector disposed on the active region such that the active region is interposed between the first reflector and the second reflector. The second reflector includes a polarizer for polarizing light generated from the active region. The polarizer contains parallel stripes of material doped differently than that of the second reflector.
Another embodiment is directed to a method for fabricating a polarizer for polarizing light in a VCSEL, which includes a substrate, a first reflector disposed on the substrate, an active region disposed on the first reflector, and a second reflector disposed on the active region such that the active region is interposed between the first reflector and the second reflector. The method includes the steps of growing a first layer of a first doped material on the active region, growing a second layer of a second doped material on the first layer, growing a third layer of the first doped material on the second layer, etching one or more grooves through the second and third layers into the first layer, and covering the etched grooves with the first doped material so as to form parallel stripes with the second layer. In an embodiment, the polarizer reflects to a relatively greater degree light with an electric (E) field vector (polarization) parallel to the stripes and reflects to a relatively smaller degree light with an E field vector perpendicular to the stripes. Because the polarizer reflects to a higher degree light having parallel polarization, it may be said to relatively reflect light with an E field vector parallel to the stripes and to relatively transmit light having an E field vector perpendicular to the stripes. In this application, therefore, to xe2x80x9crelatively reflectxe2x80x9d light with an E field vector parallel to the stripes and to xe2x80x9crelatively transmitxe2x80x9d light having an E field vector perpendicular to the stripes means that the polarizer has a higher reflectivity, and thus a lower transparency, for light with an E field vector parallel to the stripes, than for light with an E field vector perpendicular to the stripes. In another embodiment, the polarizer substantially reflects light with an E field vector parallel to the stripes and substantially transmits light with an E field vector perpendicular to the stripes.
Another embodiment in accordance with the present invention is a method for fabricating a polarizer, which includes the steps of growing a first semiconductor layer of a first doped material, growing a second semiconductor layer of a second doped material on the first semiconductor layer, growing a third semiconductor layer of the first doped material on the second semiconductor layer, etching one or more grooves through the second and third semiconductor layers into the first semiconductor layer, and covering the etched grooves with the first doped material so as to form parallel stripes with the second semiconductor layer. The polarizer relatively reflects light having an E field vector parallel to the stripes and relatively transmits light having an E field vector perpendicular to the stripes.
Another embodiment in accordance with the present invention is a polarizer that includes a semiconductor layer. The semiconductor layer has parallel stripes of material doped differently than that of the rest of the semiconductor layer. The stripes are composed of the same material as the semiconductor layer. The polarizer relatively reflects light having an E field vector parallel to the stripes and relatively transmits light having an E field vector perpendicular to the stripes.
Another embodiment is a method for polarizing light in a VCSEL, which includes a substrate, a first reflector disposed on the substrate, an active region disposed on the first reflector, and a second reflector disposed on the active region such that the active region is interposed between the first reflector and the second reflector. The method includes the steps of: generating light in the active region of the VCSEL and subjecting the light to a polarizer fabricated inside the second reflector, where xe2x80x9cinsidexe2x80x9d indicates that the polarizer is between the lower and upper ends of the second reflector. The polarizer includes parallel stripes of material doped differently than that of the second reflector.
An advantage of the present invention is that the VCSEL will emit light with a predictable and stable polarization.
Other and further features and advantages will be apparent from the following description of presently preferred embodiments of the invention, given for the purpose of disclosure and taken in conjunction with the accompanying drawings. Not all embodiments of the invention will include all the specified advantages.